nftables.service: add ConditionFileNotEmpty=/etc/nftables.conf
When booting with an empty /etc/ (factory reset, image-based build with only /usr/) nftables.service fails:
Feb 28 00:18:41 localhost nft[475]: internal:0:0-0: Error: Could not open file "/etc/nftables.conf": No such file or directory Feb 28 00:18:41 localhost systemd[1]: nftables.service: Main process exited, code=exited, status=1/FAILURE
Add a condition to the unit, so that it is skipped unless a config file actually exists, to avoid spurious failures.